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A MAS NMR investigation of aluminosilicate, silicophosphate, and aluminosilicophosphate gels and the evolution of crystalline structures on heating the gels

Published online by Cambridge University Press:  08 February 2011

S. Prabakar ,
K.J. Rao  and
C.N.R. Rao
S. Prabakar
Affiliation:
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560 012, India
K.J. Rao
Affiliation:
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560 012, India
C.N.R. Rao*
Affiliation:
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560 012, India
*
a)Address correspondence to this author.
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Abstract

Gels of various composition containing SiO2, Al2O3, and P2O5 have been investigated by employing high resolution magic-angle-spinning (MAS) 27Al, 29Si, and 31P NMR spectroscopy. Changes occurring in the NMR spectra as the gels are progressively heated have been examined to understand the nature of structural changes occurring during the crystallization of the gels. 27Al resonance is sensitive to changes in the coordination number even when the Al concentration is as low as 1 mol%. As the percentage of Al increases, the hydroxyl groups tend to be located on the Al sites while Si remains as SiO4/2 (Q4). Mullite is the major phase formed at higher temperature in the aluminosilicate gels. In the case of the silicophosphate gels, Si is present in the form of Q4 and Q3 species. There is a change in the coordination of Si from four to six as the gel is heated. The formation of six-coordinated Si is facilitated even at lower temperatures (∼673 K) when the P2O5 content is high. The phosphorus atoms present as orthophosphoric acid units in the xerogels change over to metaphosphate-like units as the gel is heated to higher temperatures. In aluminosilicophosphates, Si is present as Q4 and Q3 species while P is present as metaphosphate units; Al in these gels seems to be inducted into the tetrahedral network positions.

Type
Articles
Information
Journal of Materials Research , Volume 6 , Issue 3 , March 1991 , pp. 592 - 601
Copyright
Copyright © Materials Research Society 1991

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